1.1 Field
The field of the invention is apparatuses and methods for predicting the composition and properties of streams in a petroleum refinery. More particularly, the invention is directed to apparatuses and methods for generating on-line, near real-time, dynamically updated predictions of the composition and properties of a given stream.
1.2 Description of Related Art
Crude oil, or petroleum, is a complex multi-component mixture of various hydrocarbons and compounds containing nitrogen, oxygen and sulfur. Most crude oil also contains minor amounts of nickel and vanadium.
The exact chemical composition of a given crude oil will vary significantly depending on its point of origin. As a result, the chemical and physical properties of crude oil vary. Crude oil itself is rarely used as a fuel because (a) its properties are too variable and (b) its properties do not meet the specifications of most furnaces, boilers and engines. Processing crude oil into more useful fuels and other petroleum based products is relatively complex and refinery operations vary greatly. However, at a high level, a number of commonalities exist. First, in a typical refinery, crude oil purchased on the open market is stored and commingled in storage tanks until processing. Thus, the initial crude oil feed is often a mixture of different crude oils. Second, the feed is heated on its way to an atmospheric distillation unit (a.k.a. atmospheric pipestill or “APS”). A typical APS tower, or column, comprises a series of trays, liquid side draws, pump arounds, and a condenser. The APS tower distills the feed at atmospheric pressure and separates the feed into fractions, or cuts, each cut being equivalent in boiling range to one desired product, e.g., gasoline, naphtha, kerosene, light gas oil (LGO), heavy gas oil (HGO) and residue. Third, each fraction, or cut, is removed as a stream through a liquid side draw in the APS tower. The streams removed from the APS tower through the liquid side draws can be sold as finished products or blended with other streams and/or further processed to increase economic value. For example, high boiling APS products such as kerosene, LGO, HGO and residue can be converted into lower boiling products such as gasoline by subjecting the streams to further distillation in a vacuum pipestill (VPS) followed by thermal cracking and/or catalytic cracking in a fluid catalytic cracking (FCC) unit.
Alternatively or in addition, catalytic reforming, isomerization, alkylation, polymerization, hydrogenation, and various other operations and combinations thereof can be used to upgrade streams into higher value products.
Based on supply and demand, the value of any given petroleum product will change on a daily basis. Furthermore, different crude oils and intermediate streams, because of their composition, are better suited to make different products. Finally, the pressures, temperatures and flow rates that are ideally suited to successfully process each stream in a refinery process vary depending on the composition of the stream. Refineries are constantly seeking better ways to optimize the operation of each unit in the process to better achieve the optimal balance between the amount and value of petroleum products produced and the production expenses incurred.
In the petrochemical industry, extremely detailed analyses of feed and product materials (assays) are often utilized for making business decisions, for planning, controlling and optimizing operations and for certifying products. Chief among these analyses is the crude assay. When a crude oil is assayed, it is distilled in two steps. A method such as ASTM D2892 (see Annual Book of ASTM Standards, Volumes 5.01-5.03, American Society for Testing and Materials, Philadelphia, Pa.) can be used to isolate distillate cuts boiling below approximately 650° F. (343° C.). The residue from this distillation is further distilled using a method such as ASTM D5236 to produce distillate cuts covering the range from 650° F. to approximately 1000-1054° F. (343° C. to 538-568° C.) and a vacuum residue cut. At a minimum, cuts corresponding to typical products or unit feeds are typically isolated that include following: liquefied petroleum gases (LPG) at initial boiling point to 68° F. (20° C.); light straight run naphtha (LSR) at 68° F. to 155° F. (20° C. to 68.3° C.); heavy straight run naphtha (HSR) at 155° F. to 350° F. (68.3° C. to 176.7° C.), kerosene at 350° F. to 500° F. (176.7° C. to 260° C.), diesel at 500° F. to 650° F. (260° C. to 343.3° C.), vacuum gas oil at 650° F. to 1000° F. (343.3° C. to 537.8° C.); and vacuum residue at 1000° F. to 1054° F. (537.8° C. to 567.8° C.). Each distillate cut is then analyzed for elemental, molecular, physical and/or performance properties. The specific analyses conducted depend on the typical disposition of the cut. Example analyses are known in the art and extensively discussed and tabulated in, inter alia, U.S. Pat. No. 6,662,166.
Depending on the intended use of the crude assay data, different organizations will employ different assay strategies. For compositional and process modeling, extremely detailed analyses, mapping thousands of molecules, may be employed. For example, the high detail hydrocarbon analysis (HDHA) method described by Jacob, Quann, Sanchez and Wells (Oil and Gas Journal, Jul. 6, 1998) can be employed.
A detailed crude assay using a process, such as HDHA, costs many thousands of dollars and takes weeks to complete. As a result, the assay data used for making business decisions, and for planning, controlling and optimizing operations, is seldom from the cargoes currently being bought, sold or processed but rather historical data for “representative” past cargoes.
More recently, Exxon Mobil Corporation's virtual assay technology has dramatically lowered the time and expense to obtain a detailed crude assay to a few hours and a few hundred dollars. Virtual assay compares spectral data to a library of historical spectra and other information and is described, inter alia, in U.S. Pat. No. 6,662,116 issued Dec. 9, 2003, the entire contents of which are hereby incorporated by reference.
The crude assay data will typically be stored in an electronic database where it can be mathematically manipulated to estimate crude qualities for any desired distillation range. For example, commercial crude assay libraries are available from Haverly Systems Inc. and HPI Consultants Inc.—both of which provide tools for manipulating the data, as does Aspentech Inc. Assay data is published by Crude Quality Inc., by Shell Oil Company and by Statoil. The property versus distillation temperature data is typically fit to smooth curves that can then be used to estimate the property for any desired distillation cut.
The on-line optimum operation of the crude distillation column, blending units, and downstream process units can result in significant economic benefits. A real time optimization (RTO) system, which runs on an on-line process control computer and automatically calculates and implements the optimization results, can be used to keep each phase of the plant operation close to the economic optimum. The cornerstone of any successful RTO system is feed characterization information and the process models upon which the calculations are based. Accuracy of the process models must be sufficient to predict the actual process interactions and constraints.
Unfortunately, effective on-line RTO of crude distillation columns requires the lumping of detailed assay feed characterizations into boiling point cuts. In other words, there are about 10,000 or so different molecular species in crude oil. The model used in the RTO of the crude distillation column is unable to include all of these species. As a result, a major simplification is made. The species are grouped together into “cuts.” Each cut has a specific boiling point range. Thus, for example, all of the molecular species whose boiling point lies between 330° F. (165.6° C.) and 350° F. (176.7° C.) might be combined together into a single component known as a 340° F. (171.1° C.). This greatly reduces the number of species needed in the crude distillation column RTO. It also gives a component breakdown that is “good enough” for the crude distillation column RTO as the crude distillation column is mostly concerned with the boiling points of the components that it processes.
Therefore, detailed assay feed characterizations are unavailable to downstream RTO applications—which rely instead on static feed reference characterizations that are then tuned to current bulk properties and product yields. In other words, a static representative feed is characterized for a given downstream unit and then used as the reference for all feeds to the unit. The downstream RTO systems then tunes the static feed as needed to match current measured bulk properties, yields, or operating conditions. This static feed reference characterization, because it is not based on updated feed characterization information, often leads to sub-optimal solutions and non-robust behavior. This is becoming a problem as refineries struggle to meet stricter environmental laws and regulations governing fuel components such as sulfur.
It would be desirable to develop more dynamically updated feed characterizations for downstream process and blending RTO systems. This would permit enhanced optimization of refining processes.